Tractor trailer combinations generally require a braking system on the trailer that is engaged when a braking system of the tractor is selectively engaged. Often the braking system of the trailer is coupled to the braking system of the tractor to allow the tractor to simultaneously apply the braking system of the tractor and the trailer. Trailer brake systems often include a hydraulic component and a pneumatic component. The pneumatic component of the system may be modulated by an operator, but the hydraulic component is not.